From Fire Emblem Heroes Wiki
Light's Blessing is an item that can be used during battle, and it will restore all allies' HP, movement, and use of Specials.
- It can be used at any point during the battle, even after all of your units are defeated.
- If you try to clear a quest that states, that all four allies must survive, but one or more of them falls in battle, a Light's Blessing can be used to restore your allies and the quest will still be cleared.
- There are exceptions however, Grand Hero Battle Maps being one of them.
- If you don't have any Light's Blessings in your Inventory, it is possible to use an Orb in its place. This however is ill-advised, because orbs are much more valuable than some stamina.